Technical Field
The present invention relates to a method for controlling a wind park, as well as such a respective wind park. In particular, the present invention relates to controlling a wind park to feed electrical power into an electrical AC grid on a point of common coupling, as well as such a wind park.
Description of the Related Art
Wind parks are generally known, they relate to several wind power installations that belong together in organizational terms. In particular, all wind power installations that belong to a wind park feed into an electrical AC grid on a point of common coupling. Usually, each wind power installation itself generates an electrical current that is to be fed, i.e., usually a 3-phase electrical current that is to be fed in. For this purpose, the wind power installation refers to the voltage in the electrical AC grid that is to be fed in, which is also simply referred to below as grid, in particular according to the amplitude, frequency, and phase of the voltage.
In addition, it is meanwhile known and desirable to use wind power installations, particularly wind parks, to support the grid. In other words, the aim is not only to feed as much energy into the grid as possible, but also to feed it in such a way, and, if necessary, even to reduce the fed-in power, that the grid can be supported in electrical terms. First patent applications that have dealt with such topics are WO 02/086315, WO 02/086314, WO 01/86143, WO 99/33165, and WO 02/044560. A method is known from WO 03/030329 A1 according to which all of the output power of the wind park can be reduced externally by the operator of the connected electrical supply grid.
Furthermore, reference is made to the essay “Loss of (Angle) Stability of Wind Power Plants” by V. Diedrichs et al., submitted for and presented at the “10th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Farms, Aarhus (Denmark), 25-26 Oct. 2011”. There, reference was basically made to the problem that the loss of stability in the grid can basically also occur for wind power installations that are connected to the supply grid for feed-in.
Here, the operator can predetermine a percentage value by which the respective wind power installations can reduce their power.
Such approaches are partly already provided to stabilize the grid. In particular, these solutions consider an adjustment of the fed-in power to the current demand; in particularly, they are to consider an oversupply or undersupply of power in the grid.
When feeding in electrical power, i.e., both active power as well as reactive power, it is an overall need to secure the stability of power systems and power plants, including wind power installations and wind parks. Here, stability relates to frequency and voltage simultaneously in all areas of the power system.
The loss of such a stability is generally also referred to as “loss of stability”, and can be abbreviated as LOS. “Loss of stability” describes physical processes and conditions, which no longer secure said stability, and illustrates that these are to be avoided or stopped as soon as possible, if they already exist. These problems are basically rare, but they are therefore all the more serious. For example, this includes a generally known shut-off of portions of the grid, as occurred, for example, in 2004 in the US, or of the overall power system, as occurred in 2004 in Italy.
Basically, technical knowledge with regard to the topic of stability has been developed in depth and dealt with in a wide variety of publications. An internationally recognized standard work is Kundur, P.: Power Systems Stability and Control, McGraw-Hill.
The so-called “short circuit ratio” (SCR) serves to assess the operability of power plants on a global scale, mostly with synchronous generators, on point of common couplings with power systems.
Besides such a global or absolute assessment by means of the SCR, further assessments are conducted according to special criteria. Such criteria aim at different types of processes that are relevant to stability, such as the process of a voltage collapse, or at the stability of an angle, i.e., phase angles in the grid, which is generally referred to as “angle stability”. These assessments particularly provide metrics or standards for stability distances.
This short circuit current ratio is the ratio of the short circuit power to the connected load. Here, short circuit power is the power that the respective supply grid on the considered point of common coupling, to which the relevant power plant is to be connected, can provide in the case of a short circuit. The connected load is the connected load of the power plant that is to be connected, in particular the nominal capacity of the generator that is to be connected.
With regard to the requirements of a short circuit current ratio, SCR, a short circuit current ratio of SCR>4, however, practically often SCR>10, has been considered necessary for the reliable operation of power plants with synchronous generators. For this purpose, for Germany, reference is made to the VDN Transmission Code 2007. A short circuit current ratio of SCR>4 . . . 6 is usually required on the market for the connection of wind power installations or wind parks.
The accordingly required amount of the SCR limits the power of the power plant on a given “point of common coupling” (PCC), as it is generally referred to, or it determines required grid reinforcements.
The short circuit power is a grid characteristic on the respective point of common coupling, and thus at first a predetermined value, if the respective grid already exists there. As the short circuit current ratio should not fall short of a certain value, i.e., particularly in the area of 4 to 6, the power of a power plant or a wind park that is to be connected to a point of common coupling is limited. Therefore, power plants can only be connected up to a certain value, or it becomes necessary to expand the grid in order to facilitate the connection of a power plant with higher output.
The German Trademark and Patent Office has investigated the following state-of-the-art in the priority application: DE 10 2009 030 725 A2, WO 2011/050807 A2 and Loss of (Angle) Stability of Wind Power Plants—The Underestimated Phenomenon in Case of Very Low Short Circuit Ratio in 10th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Farms, Aarhus, 26. Oct. 2011 by Volker Diedrichs, Alfred Beekmann, Stephan Adloff.